Abstract: Aiming at the different performances of SCR system owing to different structures of the flue and the ammonia-injection grid under the requirements of ultra clean emissions, the performances of typical SCR devices were analysed and optimized by testing and adjustment. The influence of branch pipe installation on the uniformity distribution of NO_x at the exit was contrastively analysed, and the performances of ammonia spray branch pipes studied by the numerical simulation. The results showed that the relative standard deviations (RSDs) of inlet flue gas velocity distribution of the A, B sides of Test Device 1 were 12.4% and 14.4%, respectively. However, the performance and installation of the ammonia spray branch pipes before optimization could not realize in the NO_x uniformity distribution in the depth direction of the exit cross section. After optimization, the partition control in the horizontal and vertical direction of the ammonia-injection grid system resulted in the uniform mixing of ammonia-nitrogen in Test Device 1, with the RSD of its outlet speed of 6.9%. In Test Device 2, the RSDs of the inlet flue gas velocity distribution and the outlet velocity distribution of the ammonia spray branch pipes were 27.8% and 23.7%, respectively. But, the RSD of NO_x on the exit cross section was 16.3%, as a result of correct installation of ammonia spray branch pipes.